Relative analgesia is a form of conscious sedation produced by the administration of anaesthetic gas at a level insufficient to produce complete anaesthesia but sufficient to suppress the gag reflex. By this means a throat pack can be inserted into the patient's mouth so as to obstruct the oro-pharyngeal region during surgical or dental work in the patient's mouth. The anaesthetic gases normally used are a mixture of nitrous oxide and oxygen, but other anaesthetic gases could be used in other situations. In any event, the escape of anaesthetic gases into the surrounding atmosphere of the surgery or theatre is a considerable problem, as it can have noticeable effects on the skilled personnel in attendance, and the possible long term effects to such people of continual exposure to such gases is giving rise to some concern.
Conventional nasal masks are of various types. In one type the mask is provided around its face-contacting periphery with a pneumatic cuff. This is intended to be deformable so as to adapt to the facial contours of different patients. However, in practice the cuff has rather limited deformability, and moreover tends to be uniformly deformable as a result of its being a continuous inflated envelope, and hence it does not take account of the fact that there is much more variation in shape and dimensions of some facial areas, as compared with others. As a result, there is usually a very imperfect seal against the patient's face, and a considerable escape of gas results.
An example of this type of mask is the Goldman nasal inhaler (see "Aids to Anaesthesia" by Victor Goldman, published Bailliere, Tindall & Cox, London, 1948, especially at pages 98 and 99).
Another form of conventional mask does not have a pneumatic cuff, but is simply a resilient moulded rubber or plastics shape which is intended to fit closely around the nose of the patient, and whose periphery is intended to be sufficiently flexible so as to accommodate different facial shapes and dimensions in the area contacted. In practice, however, it is found that insufficient regard is paid to the greater variation of facial shapes and dimensions in some parts of the face as compared with others, and again leakage is commonly experienced. Moreover, the mask has to be held firmly against the face to minimise leakage, and is easily displaced by pressure applied to the gas inlet and outlet conduits which are located one on each side of the mask. Examples of such masks are shown in U.S. Pat. No. 1,287,149 (Walter et al), U.S. Pat. No. 2,241,535 (Boothby et al), U.S. Pat. No. 3,799,164 (Rollins) and U.S. Pat. No. 4,248,218 (Fischer). A mask resembling that of Rollins, above, is currently manufactured by Fraser Sweatman Inc., in the United States. A modification of this second type of mask is the "Brown scavenging mask", currently marketed by Narco McKesson in the United States, and includes an outer shell surrounding the mask and connected to vacuum, in an attempt to scavenge leaking gas along with exhaled gas. A third type of mask has been proposed in DE OLS 2244887 (Gmahle), where the mask is made of polymeric foam material, with a central opening for inlet and outlet of gas, and an inset reinforcement around the opening. The reinforcement extends only part way towards the face-contacting region of the mask, the latter region being formed with a thick bead of rounded cross-section, intended to form a seal against the face. No other detail is given as to the mask's shape or intended mode of use, and it is not known whether this mask was intended for anaesthesia (it is referred to as a "respiration mask"), or whether it was ever made at all. At any rate, it seems likely that it was intended to have a generally circular face-contacting opening, and to cover both the nose and the mouth, probably for the administration of oxygen (where of course leakage of gas from the mask is normally of little importance).
Another feature of conventional masks concerns the shape of their face-contacting openings. Nasal masks as currently manufactured seem all to have a high and narrow shape to this opening, somewhat bulbous towards the bottom to accommodate the base of the nose, the top of the mask extending high on the bridge of the nose, or even to the base of the forehead. Most of the above-identified prior art masks are of this kind. An exception appears to be the mask of Walter et al, which appears to have a circular face-contacting opening for both the facial and nasal masks, albeit with an indentation to receive the nose, and, in the case of the facial mask, a slight recess to accommodate the curvature of the chin.